1. Field of the Invention
This invention relates to flash units for cameras, and more particularly to automatic electronic flash units for automatically setting a proper aperture value using the reflection of a preliminary flash.
2. Description of the Prior Art
The above-described type of flash unit disclosed, for example, in Japanese Laid-Open Patent Application No. Sho 58-102221 (U.S. patent application Ser. No. 447,508 now abandoned). In flash units of this type, a correct aperture value can be obtained by preliminary illumination prior to making the flash exposure. This makes it possible to accurately control the exposure. This is particularly advantageous with a bounce flash exposure made by vertically or horizontally tilting the flash discharge tube relative to the photo-sensor that points toward an object to be photographed. There, the preliminary flash illumination tells the photographer that for the given object distance the aperture value obtained falls within the dynamic range of exposure control.
However, when the above-described type of flash unit is used in bounce flash photography, the determination of an aperture value by preliminary illumination may be followed by tilting of the flash firing portion. Then the aperture value is no longer correct. If such tilting takes place accidentally and the photographer is unaware of it before he starts to actuate the camera release, an improper flash exposure will result.